The present invention relates to solid-phase peptide synthesis (SPPS), and in particular relates to microwave-assisted techniques for SPPS.
The early part of the twentieth century saw the birth of a novel concept in scientific research in that synthetically produced peptides could greatly facilitate the study of the relationship between chemical structure and biological activity. Until that time, the study of structure-activity relationships between peptides and their biological function had been carried out using purified, naturally occurring peptides. Such early, solution-based techniques for peptide purification were plagued with problems, however, such as low product yield, contamination with impurities, their labor-intensive nature and the unpredictable solubility characteristics of some peptides. During the first half of the twentieth century some solution-based synthesis techniques were able to produce certain “difficult” peptides, but only by pushing known techniques to their limits. The increasing demand for higher peptide yield and purity resulted in a breakthrough technique first presented in 1963 for synthesizing peptides directly from amino acids, now referred to as solid-phase peptide synthesis (SPPS).
The drawbacks inherent in solution-based peptide synthesis have resulted in the near-exclusive use of SPPS for peptide synthesis. Solid phase coupling offers a greater ease of reagent separation, eliminates the loss of product due to conventional chemistries (evaporation, recrystalization, etc.), and allows for the forced completion of the reactions by adding excess reagents.
Peptides are defined as small proteins of two or more amino acids linked by the carboxyl group of one to the amino group of another. Accordingly, at its basic level, peptide synthesis of whatever type comprises the repeated steps of adding amino acid molecules to one another or to an existing peptide chain of acids.
The synthetic production of peptides is an immeasurably valuable tool in the field of scientific research for many reasons. For example, some antiviral vaccines that exist for influenza and the human immunodeficiency virus (HIV) are peptide-based. Likewise, some work has been done with antibacterial peptide-based vaccines (diphtheria and cholera toxins). Synthetically altered peptides can be labeled with tracers, such as radioactive isotopes, and used to elucidate the quantity, location, and mechanism of action of the native peptide's biological acceptor (known as a receptor). This information can then be used to design better drugs that act through that receptor. Peptides can also be used for antigenic purposes, such as peptide-based antibodies to identify the protein of a newly discovered gene. Finally, some peptides may be causative agents of disease. For example, an error in the biological processing of the beta-amyloid protein leads to the “tangling” of neuron fibers in the brain, forming neuritic plaques. The presence of these plaques is a pathologic hallmark of Alzheimer's Disease. Synthetic production of the precursor, or parent molecule, of beta-amyloid facilitates the study of Alzheimer's Disease.
These are, of course, only a few of the wide variety of topics and investigative bases that make peptide synthesis a fundamental scientific tool.
The basic principle for SPPS is the stepwise addition of amino acids to a growing polypeptide chain that is anchored via a linker molecule to a solid phase particle which allows for cleavage and purification once the coupling phase is complete. Briefly, a solid phase resin support and a starting amino acid are attached to one another via a linker molecule. Such resin-linker-acid matrices are commercially available (e.g., Calbiochem, a brand of EMD Biosciences, an affiliate of Merck KGaA of Darmstadt, Germany; or ORPEGEN Pharma of Heidelberg, Germany, for example). The starting amino acid is protected by a chemical group at its amino terminus, and may also have a chemical side-chain protecting group. The protecting groups prevent undesired or deleterious reactions from taking place at the alpha-amino group during the formation of a new peptide bond between the unprotected carboxyl group of the free amino acid and the deprotected alpha-amino of the growing peptide chain. A series of chemical steps subsequently deprotect the amino acid and prepare the next amino acid in the chain for coupling to the last. Stated differently, “protecting” an acid prevents undesired side or competing reactions, and “deprotecting” an acid makes its functional group(s) available for the desired reaction.
When the desired sequence of amino acids is achieved, the peptide is cleaved from the solid phase support at the linker molecule. This technique consists of many repetitive steps making automation attractive whenever possible.
Many choices exist for the various steps of SPPS, beginning with the type of reaction. SPPS may be carried out using a continuous flow method or a batch flow method. Continuous flow is useful because it permits real-time monitoring of reaction progress via a spectrophotometer. However, continuous flow has two distinct disadvantages in that the reagents in contact with the peptide on the resin are diluted, and scale is more limited due to physical size constraints of the solid phase resin. Batch flow occurs in a filter reaction vessel and is useful because reactants are accessible and can be added manually or automatically.
Other choices exist for chemically protecting the alpha-amino terminus. A first is known as “Boc” (N-t-butoxycarbonyl). Although reagents for the Boc method are relatively inexpensive, they are highly corrosive and require expensive equipment. The preferred alternative is the “Fmoc” (9-fluorenylmethyloxycarbonyl) protection scheme, which uses less corrosive, although more expensive, reagents.
For SPPS, solid support phases are usually polystyrene suspensions; more recently polymer supports such as polyamide have also been used. Preparation of the solid phase support includes “solvating” it in an appropriate solvent (dimethyl formamide, or DMF, for example). The solid phase support tends to swell considerably in volume during solvation, which increases the surface area available to carry out peptide synthesis. As mentioned previously, a linker molecule connects the amino acid chain to the solid phase resin. Linker molecules are designed such that eventual cleavage provides either a free acid or amide at the carboxyl terminus. Linkers are not resin-specific, and include peptide acids such as 4-hydroxymethylphenoxyacetyl-4′-methylbenzyhydrylamine (HMP), or peptide amides such as benzhydrylamine derivatives.
Following the preparation of the solid phase support with an appropriate solvent, the next step is to deprotect the amino acid to be attached to the peptide chain. Deprotection is carried out with a mild base treatment (picrodine or piperidine, for example) for temporary protective groups, while permanent side-chain protecting groups are removed by moderate acidolysis (trifluoroacetic acid, or TFA, as an example).
Following deprotection, the amino acid chain extension, or coupling, is characterized by the formation of peptide bonds. This process requires activation of the C-alpha-carboxyl group, which may be accomplished using one of five different techniques. These are, in no particular order, in situ reagents, preformed symmetrical anhydrides, active esters, acid halides, and urethane-protected N-carboxyanhydrides. The in situ method allows concurrent activation and coupling; the most popular type of coupling reagent is a carbodiimide derivative, such as N, N′-dicyclohexylcarbodiimide or N, N-diisopropylcarbodiimide.
After the desired sequence has been synthesized, the peptide is cleaved from the resin. This process depends on the sensitivity of the amino acid composition of the peptide and the side-chain protector groups. Generally, however, cleavage is carried out in an environment containing a plurality of scavenging agents to quench the reactive carbonium ions that originate from the protective groups and linkers. One common cleaving agent is TFA.
In short summary SPPS requires the repetitive steps of deprotecting, activating, and coupling to add each acid, followed by the final step of cleavage to separate the completed peptide from the original solid support.
Two distinct disadvantages exist with respect to current SPPS technology. The first is the length of time necessary to synthesize a given peptide. Deprotection steps can take 30 minutes or more. Coupling each amino acid to the chain as described above requires about 45 minutes, the activation steps for each acid requires 15-20 minutes, and cleavage steps require two to four hours. Thus, synthesis of a mere twelve amino acid peptide may take up to 14 hours. To address this, alternative methods of peptide synthesis and coupling have been attempted using microwave technology. Microwave heating can be advantageous in a large variety of chemical reactions, including organic synthesis because microwaves tend to interact immediately and directly with compositions or solvents. Early workers reported simple coupling steps (but not full peptide synthesis) in a kitchen-type microwave oven. Such results are not easily reproducible, however, because of the limitations of a domestic microwave oven as a radiation source, a lack of power control, and reproducibility problems from oven to oven. Others have reported enhanced coupling rates using microwaves, but have concurrently generated high temperatures that tend to cause the solid phase support and the reaction mixtures to degenerate. Sample transfer between steps has also presented a disadvantage.
Another problem with the current technology is aggregation of the peptide sequence. Aggregation refers to the tendency of a growing peptide to fold back onto itself and form a loop, attaching via hydrogen bonding. This creates obvious problems with further chain extension. Theoretically, higher temperatures can reduce hydrogen bonding and thus reduce the fold-back problem, but such high temperatures can create their own disadvantages because they can negatively affect heat-sensitive peptide coupling reagents. For this reason, SPPS reactions are generally carried out at room temperature, leading to their characteristic extended reaction times.